1. Field of Invention
This invention pertains to a process for preparing divalent scandium.
2. Description of Prior Art
There have been to date only three incontrovertible reports of having produced divalent scandium (Sc.sup.2+) in calcium difluoride (CaF.sub.2) crystals. These studies all identified the product as divalent scandium using the method of Electron Paramagnetic Resonance (EPR) in which divalent scandium exhibits a unique and unmistakable hyperfine structure. Several other reports have claimed to have produced this species, basing their claims on optical measurements. However, recent studies have demonstrated that these optical properties actually arise from other defects, and they cannot be used to demonstrate the presence of divalent scandium. In the three reports which employed EPR detection, two different methods were used to produce the divalent scandium species. In the earlier work, the following three step process was used: (1) scandium metal was evaporated onto the surface of a cleaved calcium difluoride crystal; (2) scandium metal deposited on the crystal was driven into bulk crystal by diffusion at high temperature; and (3) the crystal was subjected to ionizing radiation. Divalent scandium was detected only after the third step (ionizing radiation). In the more recent work, the doping was achieved by adding scandium trifluoride to the pure starting material prior to crystal growth and the trivalent scandium was then reduced to the divalent state by exposure to ionizing radiation, as in the earlier method.
Both prior art methods suffer from serious deficiencies. The doping technique employed in the earlier method limits the total scandium content to about 120 ppm or 3.times.10.sup.18 per cm.sup.3 or less. Only a small fraction of that total scandium is present in the divalent state, and much of it appears to remain as a metal colloid which causes a general optical darkening of the crystal. The more recent method produces optically clear samples but it also converts only a small fraction (typically 10.sup.-3) of the trivalent scandium to the divalent state. Furthermore, significant fading of this concentration occurs during dark storage at room temperature.